Mesoporous Co3O4@CdS nanorods as anode for high-performance lithium ion batteries with improved lithium storage capacity and cycle life

Transition metal oxides based anodes are facing crucial problems of capacity fading at long cycles and high rates due to electrode degradations. In this prospective, an effective strategy is employed to develop advanced electrode materials for lithium-ion batteries (LIBs). In the present work, a mesoporous Co3O4@CdS hybrid sructure is developed and investigated as anode for LiBs. The hybrid structure owning porous nature and large specific surface area, provides an opportunity to boost the lithium storage capabilities of Co3O4 nanorods. The Co3O4@CdS electrode delivers an initial discharge capacity of 1292 mA h g−1 at 0.1C and a very stable reversible capacity of 760 mA h g−1 over 200 cycles with a capacity retention rate of 92.7%. In addition, the electrode exhibits excellent cyclic stability even after 800 cycles and good rate performance as compared to previously reported electrodes. Moreover, density functional theory (DFT) and electrochemical impedance spectroscopy (EIS) confirm the enhanced kinetics of the Co3O4@CdS electrode. The efficient performance of the electrode may be due to the increased surface reactivity, abundant active sites/interfaces for rapid Li+ ion diffusion and the synergy between Co3O4 and CdS NPs. This work demonstrates that Co3O4@CdS hybrid structures have great potential for high performance batteries.


Introduction
With the rapid increase in the global energy crisis and environmental pollution, researchers directed their research activities to explore effective ways for store energy.Currently, nations around the world are working hard to produce clean, sustainable, and renewable energy sources including solar, wind, and ocean power.However, these sources need energy storage systems to regulate energy production.So far various energy storage devices such as batteries, supercapacitors and fuel cell etc. have been fabricated to store energy from renewable sources.Among these devices, rechargeable lithium-ion batteries (LIBs) is crucial to meet future needs for industries from personal devices to automobiles. 1 However, durability and energy densities of current LIBs are restricted by electrode material. 2Therefore, it remains a great challenge to develop advanced electrode materials for LIBs that exhibits improved cycling stability and larger specic capacities.The current progress in the eld of energy storage materials provides more innovative solutions to resolve the energy storage issues.
So far, various transition metal oxides (TMOs) such as MnO 2 , 3 V 2 O 5 , 4 ZnO, 5 WO 3 , 6 Co 3 O 4 , 7 etc. have been exploited as the anode materials for LIBs.Among these, cobalt oxide (Co 3 O 4 ) is regarded as one of the most promising anode materials due to its high theoretical capacity (∼890 mA h g −1 ), environmental friendliness, and superior electrochemical properties.However, its poor cyclic stability, irreversible capacity loss and slow kinetics of Li-ion and electron transport hinder its practical application.In order to overcome these drawbacks, many attempts have been made such as the use of conductive polymers, 8 doping with transition metals, 9 and making composites. 10On the other hand, recent work on metal sulphides has drawn considerable attention for being the most promising electrode materials for LIBs due to their high electrical conductivity, thermal durability and rich redox chemistry than their metal oxides equivalents.To explore novel materials for energy storage devices, metal sulphides provide better option due to its outstanding features.Among various metal sulphides, cadmium sulde (CdS) has received less attention as an electrode material for LIBs.Therefore, in order to explore the energy storage features of Co 3 O 4 and CdS in a single system, their combination is found to be a good approach for the development of novel electrode material.Such hybrid systems are very appealing for energy storage applications due to their porous nature and ability to absorb guest species such as lithium ion on their surfaces and in the pore spaces.Up till now, according to our knowledge, there have been no reports found on the investigation of Co 3 O 4 @CdS nanorods as an electrode material for LIBs.For example, D. S.  CdS Hollow Spheres by the template-removal method with the assistance of the ZIF-67 material and obtained high phenol and dye photodegradation activity. 14n this work, mesoporous Co 3 O 4 @CdS nanorods are developed and investigated as an anode material for LIBs.The hybrid structure shows enhanced physical and chemical properties superior to a single counterpart.The porous nature, large specic surface area and excellent kinetics of the synthesized nanorods leads to rapid lithium storage.The developed electrode exhibits enhanced cycling stability and high-rate capability as compared to pristine Co 3 O 4 nanorods.Therefore, Co 3 O 4 @CdS structure can be considered suitable candidate as an anode material for LIBs.

Experimental
The experimental detail is given in the (ESI).†
In order to understand the vibrational behaviour of the prepared structures, Raman spectra of Co 3 O 4 and Co 3 O 4 @CdS structures were recorded in the range of (100-1000) cm −1 as shown in Fig. 1(b).In Co 3 O 4 spectrum, the bands located at 185, 478, 520, and 686 cm −1 are associated to F 2g , E g , F 2g and A 1g modes respectively. 17In Co 3 O 4 @CdS spectrum, besides Co 3 O 4 bands, two characteristic bands of CdS at 300 cm −1 and 600 cm −1 were also recorded which are attributed to the fundamental longitudinal optical phonon (1LO) and its overtone longitudinal phonon (2LO) respectively. 18In comparison with Co 3 O 4 spectrum, a small peak located at 413 cm −1 is also observed, which ascribed to the Co-S bond formation on the surface of Co 3 O 4 @CdS nanorods. 19With the close observation, a slight shi was also observed in the position of 1LO and 2LO modes which is due to the shape of the nanostructures. 20In addition, the composite spectrum exhibits broadening associated with the slight changes in the crystalline structure of pristine Co 3 O 4 as mentioned by previous reports. 21o examine the morphology of the as-prepared Co 3 O 4 @CdS structure, SEM was performed.S6 (ESI).† The calculated average pore size is ∼11 nm.The interconnected primary nanoparticles and the void spaces within a single nanorod is the primary sources for the formation of mesopores.It is wellknown that the porous structure with a high electroactive surface area, plays a vital role in the electrochemical processes. 27Thus, the provision of such good mesoporous surface, accompanying abundant active sites, proves to be benecial for lithium ion battery.
3.2.Electrochemical performance of Co 3 O 4 @CdS nanorod/ electrode 3.2.1.Cyclic voltammetry.The electrochemical performance of the assembled cells is recorded by conducting CV curves.Fig. 3(a) depicts CV curves of Co 3 O 4 @CdS cell for the 1st, 3rd and 5th cycles measured in a voltage range of 0.01-3.0V (vs.Li/Li + ) at a scan rate of 0.5 mV s −1 .During the rst cycle, cathodic peaks at 1.4, 1.0 and 0.6 V are ascribed to alloying processes, irreversible reactions and formation of solid electrolyte interphase (SEI) layer.In the 1st anodic scan two oxidation peaks at 1.9 and 2.3 V (vs.Li/Li + ) are associated to the multistep dealloying process.In the subsequent cycles, the cathodic peaks at 1.4 and 0.65 V are related to the reduction of cadmium and cobalt to their metallic states while anodic peaks at 1.9 and 2.3 V correspond to the formation of CdS and Co 3 O 4 and partial decomposition of SEI.The subsequent cycles exhibit no further change in the shape of CV curves.These results show the good reversibility of the Co 3 O 4 @CdS hybrid structure.The CV curves of the Co 3 O 4 cell for the initial few cycles are also performed under the same condition as shown in Fig. S7 (ESI).† 3.2.2.Galvanostatic charge/discharge.Fig. 3(b) shows the galvanostatic charge/discharge behaviour of Co 3 O 4 @CdS cell for the 1st, 2nd, 5th, 10th and 20th cycles at a rate of 0.1C (1C = 891 mA g −1 ) in the voltage window of 0.01-3.0V (versus Li + /Li) at room temperature.As observed, the voltage proles of the cell show a plateau at 1.2 V and rapidly reaches a plateau at 0.75 V during the rst discharge.These plateaus exhibit the conversion of Co 3 O 4 to CoO (or LixCo 3 O 4 ) and Co respectively.The initial charge and discharge capacity values are 1110 and 1292 mA h g −1 respectively with a coulombic efficiency of 76%.These capacity values are higher than the theoretical capacity of Co 3 O 4 (∼891 mA h g −1 ) and also the previously reported electrode materials as shown in Table 1.Moreover, the discharge capacity of the Co 3 O 4 @CdS electrode is found to be 995, 798, 685 and 598 mA h g −1 at 0.1C in the subsequent cycles of 2nd, 5th, 10th and 20th respectively.These values are signicantly higher than that of the Co 3 O 4 electrode.In addition, the shape of the charge/discharge cycles remain similar which show the stability of the hybrid structure as anode.For comparison, the voltage proles of Co 3 O 4 electrode is also measured as shown in Fig. S8 (ESI).† The initial charge and discharge capacity of Co 3 O 4 electrode is found to be 505 mA h g −1 and 835 mA h g −1 respectively.These capacity values are much smaller than Co 3 -O 4 @CdS cell.The improved performance of the Co 3 O 4 @CdS electrode may be due to the formation of metal sulfur bond that bridge to accelerate charge transfer between the metal oxide and sulphide.
The rate performance of both cells was also investigated.Fig. 3(c) exhibits the comparison of rate performance of Co 3 O 4 and Co 3 O 4 @CdS cells at current rates ranging between 0.1C to 0.5C.As the current rates increase from 0.1C to 5C, it can be observed that the reversible capacity of the Co 3 O 4 @CdS electrode steadily decreases from 760 mA h g −1 to 258 mA h g −1 .
Comparatively, the reversible capacities of the Co 3 O 4 electrode drop sharply from 620 mA h g −1 to 101 mA h g −1 at the similar rates (0.1C to 5C).Interestingly, when the current rate retunes at  3(d).As observed, both electrodes exhibit good cycling performance.Aer 200 cycles, the Co 3 O 4 @CdS electrode shows the reversible capacity of 760 mA h g −1 with a capacity retention rate of 83.7% and 92.7% respectively.In comparison, the reversible capacity of Co 3 O 4 electrode is 580 mA h g −1 which is much smaller that Co 3 -O 4 @CdS electrode.In order to further evaluate the long cyclic stability of the Co 3 O 4 @CdS electrode, galvanostatic charge/ discharge are performed for 800 cycles at 0.2C as shown in Fig. 3(e).It can be observed that aer 800 cycles, the electrode shows a stable reversible capacity of 520 mA h g −1 corresponding to the 90% of the initial capacity.The improved and stable performance of the Co 3 O 4 @CdS electrode is associated with its mesoporous nature which provides more active sites for Li + insertion/extraction process.These ndings demonstrate that the Co 3 O 4 @CdS structure is a suitable choice for the construction of high-rate performance batteries.
3.2.3.Electrochemical impedance spectroscopy.To investigate the kinetic behaviours and charge transfer resistance of the electrodes, EIS study is performed.Fig. 3(f) present the comparative Nyquist plots of the Co 3 O 4 @CdS and Co 3 O 4 electrodes.It can be observed, in the high frequency region, the plot of Co 3 O 4 @CdS electrode exhibits small diameter of the semicircle as compare to Co 3 O 4 electrode.This illustrate a low charge transfer resistance (135.1 U) and fast transport during the electrochemical process between electrode material and electrolyte.To nd the kinetic parameters, the experimental results are tted well by employing the equivalent circuit model as shown in the inset.The tting parameters of the two electrodes are presented in Table 2, which shows the excellent performance of the Co 3 O 4 @CdS electrode.

DFT calculations
To further understand the experimental ndings, DFT calculations is performed.The optimized structure for Co 3 O 4 (111)/CdS (002) interface is shown in Fig. 4(a).Evidently, there is a strong interaction at the interface between Co 3 O 4 and CdS (002) atoms, dominated by Co-S and Cd-S bonds.The average Co 3 O 4 (111)-CdS (002) separation is ∼2.30Å suggesting the presence of chemical bonding besides weak vdW interactions.Besides covalent bonding, interfacial interactions also have ionic  character; Bader analysis 28 indicates that there is charge transfer of ∼0.65e from Co 3 O 4 to CdS.These ndings are consistent with the recent experimental work where both the presence of Co-S interfacial bonds as well as an increase (reduction) in electronic density was observed for CdS (Co 3 O 4 ) surface within Co 3 O 4 @CdS. 29he impact of strong interfacial interactions on electronic structure is highlighted in Fig. 4(b) where density of states (DOS) contributions of Co 3 O 4 and CdS within Co 3 O 4 @CdS are presented.Interestingly, CdS has states around Fermi level (E F ) suggesting a metallic behaviour.This is in contrast to pristine CdS layer which has a large band gap of ∼2.0 eV Fig. S9 (ESI).† Therefore, it is evident that transport character of CdS layer on Co 3 O 4 changes from semiconducting to metallic, owing to strong interfacial interactions.Moreover, conductivity of Co 3 -O 4 @CdS will be higher as compared to the individual pristine surfaces, especially close to the interface region.Overall, both the increase in conductivity of Co 3 O 4 @CdS as well as higher electronic density of CdS due to interfacial charge transfer will improve the reaction kinetics at the electrode. 30This is indeed observed in the impedance spectroscopy (Table 2) where a noticeable reduction in charge transfer resistance (R ct ) is observed for Co 3 O 4 @CdS in comparison to that of pristine Co 3 O 4 .
The improved lithium storage performance of the Co 3 O 4 @-CdS hybrid structure may have following possible reasons (i) the functionalization of CdS NPs increase the surface reactivity and porosity which creates abundant active sites/interfaces for the rapid diffusion and transportation of Li + ions during the electrochemical reaction (ii) the addition of CdS NPs increase the contact area between electrolyte and electrode and protect the Co 3 O 4 nanosheets from degradation during the charge and discharges process.(iii) The synergy between Co 3 O 4 and CdS NPs greatly improved the kinetics of Co 3 O 4 @CdS nanosheets for fast lithium storage.

Conclusion
In summary, a novel mesoporous Co 3 O 4 @CdS hybrid structure were successfully synthesized by employing hydrothermal along with SILAR method.The hybrid structure demonstrates several structural features including the large specic surface area, abundant active sites and excellent kinetics.The developed electrode showed improved lithium storage performance as compare with pristine and previously reported electrodes.The electrode delivers a stable reversible capacity of 760 mA h g −1 at 0.1C over 200 cycles with a capacity retention rate of 92.7%.The electrode can achieve reversible capacity of 520 mA h g −1 at 0.2C even aer 800 cycles.The improved lithium storage performance may be due to the increase in kinetics, synergy and abundant active sites that facilitate the storing of more Li + ions and fast transportation during the lithiation/delithiation.It is suggested that the electrochemical performance of Co 3 O 4 @CdS hybrid structure can be further enhanced by structural engineering and optimizing the amount of CdS NPs.This work provides a novel platform to construct a LIBs with long cyclic stability and high-rate capability.

Fig. 1 (
c) shows the SEM images of Co 3 O 4 @CdS nanorods.As observed from the images, the Co 3 O 4 nanorods are composed of highly dense CdS NPs interconnected with one another in uniformly ordered arrays.For comparison, the SEM images of the pristine Co 3 O 4 nanorods and Co 3 O 4 @CdS nanorods are also recorded as shown in Fig. S1 (ESI), S2(a and b) † respectively.The surface of the nanorods appear to be uniform and smooth.Fig. 1(d) represents the corresponding EDX spectrum of Co 3 O 4 @CdS which consists of Cd, S, Co, and O peaks, conrm the formation of Co 3 O 4 @CdS nanorods.The existence of Cd and S peaks further conrms the successful deposition of CdS NPs.In order to clarify the detail structure and elemental distribution of the prepared composite, EDX elemental mapping are performed.Fig. S3(a-f) † displays the STEM images of Co 3 O 4 @CdS structure and a corresponding elemental mapping of Co, O, Cd and S respectively.The presence of Cd and S conrms the deposition of CdS and no other impurities formed other than Co 3 O 4 and CdS.The detailed structural analysis was conducted via TEM and HRTEM.Fig. 1(e) exhibits a low magnication TEM image of Co 3 O 4 @CdS structure, demonstrating the deposition of CdS NPs on the whole surface of Co 3 O 4 nanorods.Fig. 1(f) displays the HRTEM analysis of the Co 3 O 4 @CdS structure and exhibits a polycrystalline nature.The well resolved lattice fringes of 0.24, 0.47 are observed corresponding to (311) and (111) planes of Co 3 O 4 nanorods.The lattice fringe of 0.33 nm belongs to (002) plane of CdS is also identied.FTIR analysis was carried out to examine and investigate the structural molecular changes and presence of different functional groups in as-prepared Co 3 O 4 @CdS structures.Fig. S4 (ESI) † illustrates the FTIR spectra of the Co 3 O 4 and Co 3 O 4 @CdS structures.The bands sited at 555 cm −1 and 657 cm −1 reveals the stretching vibrations which show the presence of Co-O bonding.The band located at 1039 cm −1 is illustrating the presence of Cd-S interaction and manifests the formation of the Co 3 O 4 @CdS composite.A sharp peak sited at 1458 cm −1 represents the bending vibrations of monodispersed Co 3 O 4 structure.Furthermore, the signicant peaks sited at 2851 cm −1 and 2918 cm −1 shows the symmetric and asymmetric stretching modes of -CH 2 which comprises from HMT which plays a signicant role as a nucleation controlled reagent and thus, favouring the surface modication of Co 3 O 4 nanorods.The surface of the nanostructures plays an important role in the electrochemical performance of the material.Therefore, XPS of Co 3 O 4 @CdS is measured to examine the surfacelocalized information of the elements.Fig. 2(a) shows the deconvoluted XPS spectrum of Co 2p, which consists of two regions at 796.7 eV and 780.9 eV corresponding to Co 2p 1/2 and Co 2p 3/2 states respectively.In the Co 2p 3/2 region, two coexisting peaks at 781.1 eV and 783.0 eV can be seen which conrms the Co 3+ and Co 2+ states.Moreover, Co 2p 1/2 region also composed of two peaks at 797.3 eV and 798.3 eV related to Co 3+ and Co 2+ states.Furthermore, two satellites peaks at 786.7 eV and 803.3 eV corresponding to Co 2p 3/2 and Co 2p 1/2 regions, reconrms the formation of Co 2+ and Co 3+ states and agrees well with the previous report. 22For comparison, XPS spectra of Co 2p region of Co 3 O 4 structures is also recorded as shown in Fig. S5 (ESI).† The area ratio of Co 3+ to Co 2+ of both structures are calculated.As observed the relative area ratio of Co 3 O 4 @CdS (1.14) is signicantly higher as compare to Co 3 O 4 (1.07) con-rming that the Co 3+ states are more exposed on the surface of Co 3 O 4 @CdS structure.It is already reported that the nanostructure with dominant Co 3+ sites exhibit superior electrochemical performance. 23-25Thus Co 3 O 4 @CdS structure with dominant Co 3+ active sites are considering more suitable for energy storage applications.Fig. 2(b) depicts the high-

Fig. 1
Fig. 1 (a) XRD patterns of pure Co 3 O 4 and Co 3 O 4 @CdS structures.(b) Raman spectra of Co 3 O 4 and Co 3 O 4 @CdS NPs structures prepared through deposition by modified SILAR method.(c) FE-SEM image of Co 3 O 4 @CdS structures; (d) corresponding EDX spectrum of Co 3 O 4 @CdS structures (e) TEM image of Co 3 O 4 @CdS structure (f) HRTEM of Co 3 O 4 @CdS structure.

Fig. 3
Fig. 3 (a) CV curves of the Co 3 O 4 @CdS electrode for 1st, 3rd and 5th cycle at a scan rate of 0.5 mV s −1 .(b) Galvanostatic charge/discharge profiles of the Co 3 O 4 @CdS electrode (c) rate performance (d) performance of electrodes at 0.1C for 200 cycles and (e) cyclic stability of Co 3 O 4 @CdS electrode at 0.2C for 800 cycles (f) comparison of the EIS curves of Co 3 O 4 @CdS and Co 3 O 4 electrodes; inset is the kinetic parameters of both electrodes.

Table 2 Fig. 4
Fig. 4 (a) Schematic description of the optimized Co 3 O 4 (111)/CdS (002) interface.Colours; blue (Co), red (O), purple (Cd) and yellow (S), (b) density of states (DOS) contributions of Co 3 O 4 and CdS surfaces within Co 3 O 4 @CdS hybrid structure.DOS is normalized to the total number of atoms within each surface to provide a clear comparison.Dashed vertical line presents Fermi level (E F ).
Patil, et al., have reported the use of the core-shell structure of Co 3 O 4 @CdS for high-performance supercapacitors, 11 F. Q. Liu, et al., prepared Co 3 O 4 /CdS photoelectrode for photoelectrochemical cathodic protection in the dark 12 and Z. Qin et al., have developed Co 3 O 4 /CdS p-n heterojunction for enhancing photocatalytic hydrogen production. 13C. Jiang et al., prepared Co 3 O 4

Table 1
Performance comparisons of various Co